This invention relates to a tasty yeast extract and its production.
Conventionally yeast extract has been produced by an autolytic method in which yeast cells are allowed to autolyze or by a hydrolytic method in which yeast cells are hydrolyzed with acid, alkali or hydrolase. The autolytic method generally comprises hydrolysis of yeast cells by the enzymes present within the yeast cells in the presence of a so-called accelerator such as an organic solvent (e.g. toluene and ethyl acetate) and common salt. However the conventional methods do not attempt to effectively utilize the ribonucleic acid present in yeast cells, so that ribonucleic acid is mostly decomposed into non-tasty compounds and thus tasty 5'-ribonucleotide is hardly found in the product.
As for the hydrolysis of yeast cells with hydrolase, a method is known in which an extract of ribonucleic acid obtained by treatment of yeast cells with an alkali or common salt, or yeast cells themselves are treated with a nuclease derived from microorganisms, animals, or plants, so that ribonucleic acid may be utilized as 5'-ribonucleotide (Japanese patent application Laid-Open No. 109153/84). However such a method is disadvantageous because the addition of enzymes derived from other species of organisms is required which makes the process complicated and less economical.
On the other hand, methods for extraction and elimination of ribonucleic acid in cells of microorganisms have been developed for utilization of microbial protein as a foodstuff or feed. A method is known in which a specified yeast of genus Candida is heated for a short time, i.e. at 60.degree. to 70.degree. C. for 5 to 20 seconds, followed by warming at 45.degree. to 50.degree. C. at pH 4.5 to 7.0 for about 20 minutes (U.S. Pat. No. 3,720,585). However, by this method ribonucleic acid is decomposed into non-tasty 3'-ribonucleotide, therefore this method is not appropriate for production of yeast extract. Another method is also known for production of microbial protein with a low content of nucleic acid in which cells are heated at 63.degree. to 67.degree. C. at pH 7 to 8.5 for 20 minutes to 20 hours so that ribonucleic acid may be decomposed to give 5'-ribonucleotide (Japanese Examined patent application No. 37799/81). However, because the enzymes required for autolysis may be inactivated by the conditions of treatment, this method is also inappropriate for production of yeast extract.
The method of production of yeast extract by autolysis is desirable for production of natural seasonings because it makes use of the action of enzymes present in the starting material yeast. However, as described above, the extract obtained by the conventional procedures of autolysis has an unsatisfactory taste with respect to body or flavor, because not only does the extract contain a 5'-ribonucleotide content of 0.5% at best and usually less than 0.1%, but the content of other tasty ingredients is also low. Hence, a satisfactory method for the production of a tasty yeast extract has not been available.